A convicted sex offender suspected of murdering at least seven women and leaving the bodies in abandoned houses in northwest Indiana may be a serial killer who has killed others in the state as far back as 20 years ago, local police said on Monday.

Darren Vann, 43, of Gary, Indiana, was charged in Lake County with one of the seven murders, according to online court records. He was being held in custody, said John Doughty, police chief of the neighboring city of Hammond.

Vann, a registered sex offender from a 2008 assault in Texas, was arrested in Gary on Saturday, a day after police were led to the slain body of 19-year-old Afrika Hardy in a Motel 6 in Hammond.

Doughty said Hardy had advertised sexual services on the classified ads website Backpage.com, and that she and Vann met at the motel. When Hardy did not return from the appointment, a woman who had helped to arrange the encounter went to track her down and found her dead from strangulation.

Once in custody Vann was cooperative, admitted his involvement in Hardy’s murder and led police to the bodies of six other women, all in abandoned houses in Gary, Doughty said.

“It could go back as far as 20 years based on some statements we have and that’s yet to be corroborated,” Doughty said. “It is possible other victims could surface.”

Doughty said the victims included Anith Jones, 35, of Merrillville, Teiarra Batey, 28, of Gary, and Christine Williams, 36, also of Gary. Three other victims have yet to be identified.

Doughty was unable to say whether the other victims were sex workers.

Jones was reported missing on Oct. 8, triggering a search for her in Gary, a city on the southern tip of Lake Michigan, 30 miles (50 km) south of Chicago.

Abandoned houses overgrown with trees and weeds are a prominent feature of Gary’s landscape – as they are in much of the U.S. rust belt of formerly well-off manufacturing and steel towns that have fallen on hard times. More than 35 percent of Gary’s residents live under the federal poverty level and the population has dropped to 78,000 from more than twice that in the 1960s.

“As we find buildings are open, we do in fact board them up. We are in the process of demolishing buildings. But when you have 10,000 buildings you don’t have the resources to demolish all of the ones that would be candidates,” Gary Mayor Karen Freeman-Wilson told reporters.

Doughty said it was not known if the victims were murdered in the abandoned houses or if their bodies were dumped there.

Vann lived in a ranch-style home in Gary with his brother and his brother’s girlfriend, said a neighbor who asked not to be identified. The neighbor said he was shocked that Vann was a suspect and that he was a nice person who took care of his brother’s young children.

Another neighbor, Latonya Ramson, 36, said abandoned homes in the neighborhood have long been a concern.

“It makes me very uncomfortable. There’s abandoned houses everywhere you look. If we didn’t have all these abandoned houses, he wouldn’t have been able to leave people in them, that’s part of the problem,” she said.

Authorities in Gary and in Austin, Texas, said they were reviewing missing persons cases to try to identify the bodies or other potential victims.

Freeman-Wilson said some of the bodies were significantly decomposed but not skeletal. Doughty said the victims known so far were killed more recently than two decades ago.

Vann was sentenced to five years and served a little more than one year after he was convicted of sexually assaulting and trying to strangle a 25-year-old woman in Austin, Texas Department of Criminal Justice spokesman Jason Clark told Reuters.

On the on-line Texas Department of Public Safety sex offender registry, Vann’s risk level was listed as “low.”

The Chicago Tribune reported that Vann’s former wife said he had served in the Marines. Yvonne Carlock at the U.S. Marine Corp’s manpower and reserve affairs office told Reuters that Darren Vann had contracted into the Marines but she was unable to immediately confirmed whether he completed basic training.